Farmed grayling, Thymallus thymallus (L.), are susceptible to atypical Aeromonas salmonicida (aAS) infections. Interactions between bacteria and parasites were studied using grayling subjected to concomitant exposure to aAS bacteria and the digenean parasite Diplostomum spathaceum. Atypical AS was detected from fish by a combination of bacterial cultivation and polymerase chain reaction techniques. A detection level of 17 aAS cells per 100 mg intestine tissue sample was obtained. Concomitant bacterial exposure did not enhance the severity of grayling eye rupture and nuclear extrusion induced by D. spathaceum, but D. spathaceum invasion into grayling increased the proportion of fish carrying aAS in their heart tissue. However, the number of aAS cells detected in heart tissue was low. Atypical AS did not cause acute disease or mortality during 15 days post-exposure. There was a higher prevalence of aAS in grayling heart samples than in intestinal samples, indicating that the intestine is not favoured by aAS. We suggest that heart tissue would be a good organ from which to isolate aAS when tracing latent carrier fish. We conclude that penetrating diplostomids can enhance bacterial infections in fish and that diplostomids can cause serious eye ruptures in grayling.

Thirty-one isolates of Saprolegnia sp., most originating from infected salmon or trout, were characterised genetically and physiologically. The majority (6 of 31) of the isolates from several widely separated geographical locations was found to be genetically almost identical as assessed by RAPD-PCR. The remaining isolates belonged to 3 different groups with 1 to 3 representatives each. It is suggested that the first group of isolates represents a virulent form of the organism that has been widely spread by clonal propagation. The ability to repeated zoospore emergence, as an alternative to direct germination, seems to characterise specific Saprolegnia genotypes that may have adapted to certain hosts.KEY WORDS: Saprolegnia · Saprolegniosis · Fish disease · Random amplification of polymorphic DNA RAPD · Repeated zoospore emergence Resale or republication not permitted without written consent of the publisherDis Aquat Org 53: [47][48][49][50][51][52][53] 2003 guished the fish lesion isolates from the water isolates, and Beakes (1983) was able to identify S. parasitica from S. diclina. Variation in esterase isoenzyme patterns (Beakes & Ford 1983) and differences in radial growth rate (Willoughby & Copland 1984, Hatai et al. 1990) have been used for determining distinct groups of fish lesion isolates. Restriction fragment length polymorphisms (RFLPs) are useful for classification of Saprolegnia and could distinguish S. parasitica from S. diclina (Molina et al. 1995). Also, random amplification of polymorphic DNA polymerase chain reaction (RAPD-PCR;Welsh & McClelland 1990, Williams et al. 1990) has been applied for analysis of the fish pathogenic Saprolegnia genome (Diéguez-Uribeondo et al. 1996, Bangyeekhun et al. 2001). This latter method provides a sensitive and rapid assay for the assessment of genetic distance between different isolates.In the present study, we applied the RAPD-PCR technique and the presence or absence of repeated zoospore emergence to characterise Saprolegnia sp. isolates obtained from Finland and Sweden to investigate the epidemiology of the saprolegniosis in this region. We found that the majority of the isolates belonged to a single genetically defined group that probably has been widely spread by clonal propagation. MATERIALS AND METHODSSaprolegnia strains. Thirty-one isolates of Saprolegnia spp. were isolated from infected tissue of the brown trout Salmo trutta m. lacustris, trout S. trutta, whitefish Coregonus lavaretus, rainbow trout Oncorhynchus mykiss, brook trout Salvelinus fontinalis, landlocked salmon Salmo salar m. sebago, salmon S. salar, noble crayfish Astacus astacus, and pond water from 6 different locations in Finland and 1 location in Sweden. The isolates from Finland are designated FinX, where X is the isolation number and the isolates from Sweden are designated Swe203 and Swe239 (see Table 1). The following reference isolates were used for comparison: S. parasitica (Spt) isolated from freshwater crayfish Astacus leptodactylus (Söderhäll et al. 1991), S. parasit...

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Atypical Aeromonas salmonicida (AAS) causes generalized lethal infections in farmed Arctic charr, Salvelinus alpinus (L.), and European grayling, Thymallus thymallus (L.), and is thus a serious threat for culture of these fish species. Virulence factors were studied among isolates of AAS from Arctic charr (n = 20), European grayling (n = 19) and other fish species (n = 20), of which 48 were of Finnish and 11 of Swedish origin. All isolates produced an A-layer. Extracellular products (ECP) of the AAS isolates did not produce detectable gelatinase and caseinase activity in test assays. Analysis of the same ECP preparations with substrate sodium dodecyl sulphate-polyacrylamide gel electrophoresis showed weak proteolytic activity, indicating the different sensitivity of the detection methods used. The ECP from AAS isolates showed low cytotoxic activity against cultured cells. However, the ECP did not induce mortality in challenged Arctic charr. The results suggest that toxic components, like ECP, secreted by the bacterium may not be the major virulence factor in AAS-infection in Arctic charr and European grayling, and hence the pathogenesis also differs from the pathogenesis of AAS-infection in Atlantic salmon, Salmo salar L.

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